1. Field of the Invention
The present invention relates to a transmission control means in an automatic transmission which has a plurality of gear change steps and which is provided with an oil temperature increase prevention mode which automatically shifts gears on the basis of a gear shift pattern of that mode, when the oil temperature of the automatic transmission rises.
2. Description of Prior Art
In an automatic transmission provided with a torque converter, the oil temperature is increased by heat generated by slip losses of the torque converter, and this high-temperature oil adversely affects the durability (frictional engaging members, etc.) and sealing properties of the transmission. For example, when a vehicle is heavily loaded (when climbing a hill, etc.), the temperature of its transmission increases, and can even reach such a high temperature that it adversely affects the transmission, as described hereinafter. In this case, a downshift increases the speed ratio e=(N.sub.T /N.sub.E) of the torque converter, as well as its efficiency, as shown in FIG. 2, and thus reduces the slip losses of the torque convertor, resulting in a prevention of the increase in oil temperature. However, the automatic transmission is provided with a certain amount of hysteresis, as shown in FIG. 6 (the hatched portion), to prevent frequent and alternate generations of upshifts and downshifts (hunting). Therefore, the downshift points are set at lower speeds than upshift points, at sufficient spacing, so that it is difficult to immediately shift gear downwardly. Therefore, when a vehicle is loaded, it is a fact that, in general, the oil temperature is likely to increase. This phenomenon cannot be removed from the conventional design concept that the automatic gear-change control of a transmission is provided with hysteresis. This problem occurs with large vehicles with which are often heavily loaded, freight vehicles, and vans, or when going up hills, regardless of the type of vehicle.
An object of the present invention is to solve this problem of the prior art. Namely, it is an object of the present invention to improve the automatic transmission so as to enable the maintenance of easy operating ability and controllability during driving, while preventing any excess increase in oil temperature.